The present invention relates to a method for producing metal-coated palladium or palladium-alloy particles useful as, for example, oxygen-reducing electrocatalysts in fuel cells. The invention particularly relates to methods for producing platinum surface monolayer nanoparticle composites having low platinum loading coupled with high catalytic activity.
A “fuel cell” is a device which converts chemical energy into electrical energy. In a typical fuel cell, a gaseous fuel such as hydrogen is fed to an anode (the negative electrode), while an oxidant such as oxygen is fed to the cathode (the positive electrode). Oxidation of the fuel at the anode causes a release of electrons from the fuel into an external circuit which connects the anode and cathode. In turn, the oxidant is reduced at the cathode using the electrons provided by the oxidized fuel. The electrical circuit is completed by the flow of ions through an electrolyte that allows chemical interaction between the electrodes. The electrolyte is typically in the form of a proton-conducting polymer membrane that separates the anode and cathode compartments and which is also electrically insulating. A well-known example of such a proton-conducting membrane is NAFION®.
A fuel cell, although having components and characteristics similar to those of a typical battery, differs in several respects. A battery is an energy storage device whose available energy is determined by the amount of chemical reactant stored within the battery itself. The battery will cease to produce electrical energy when the stored chemical reactants are consumed. In contrast, the fuel cell is an energy conversion device that theoretically has the capability of producing electrical energy for as long as the fuel and oxidant are supplied to the electrodes.
In a typical proton-exchange membrane (PEM) fuel cell, hydrogen is supplied to the anode and oxygen is supplied to the cathode. Hydrogen is oxidized to form protons while releasing electrons into the external circuit. Oxygen is reduced at the cathode to form reduced oxygen species. Protons travel across the proton-conducting membrane to the cathode compartment to react with reduced oxygen species forming water. The reactions in a typical hydrogen/oxygen fuel cell are as follows:
Anode:

Cathode:

Net Reaction:

In many fuel cell systems, a hydrogen fuel is produced by converting a hydrocarbon-based fuel such as methane, or an oxygenated hydrocarbon fuel such as methanol, to hydrogen in a process known as “reforming”. The reforming process typically involves the reaction of either methane or methanol with water along with the application of heat to produce hydrogen along with the byproducts of carbon dioxide and carbon monoxide.
Other fuel cells, known as “direct” or “non-reformed” fuel cells, oxidize fuel high in hydrogen content directly, without the hydrogen first being separated by a reforming process. For example, it has been known since the 1950's that lower primary alcohols, particularly methanol, can be oxidized directly. Accordingly, a substantial effort has gone into the development of the so-called “direct methanol oxidation” fuel cell because of the advantage of bypassing the reformation step.
In order for the oxidation and reduction reactions in a fuel cell to occur at useful rates and at desired potentials, electrocatalysts are required. Electrocatalysts are catalysts that promote the rates of electrochemical reactions, and thus, allow fuel cells to operate at lower overpotentials. Accordingly, in the absence of an electrocatalyst, a typical electrode reaction would occur, if at all, only at very high overpotentials. Due to the high catalytic nature of platinum, supported platinum and platinum alloy materials are preferred as electrocatalysts in the anodes and cathodes of fuel cells.
However, platinum is a prohibitively expensive precious metal. In fact, the required platinum loading using current state-of-the-art electrocatalysts is still too high for commercially viable mass production of fuel cells.
Accordingly, some research has focused on reducing the amount of costly platinum in fuel cell cathodes and anodes by combining the platinum with a lower cost metal. For example, U.S. Pat. No. 6,670,301 B2 to Adzic et al. relates to the deposition of ultrathin layers of platinum on ruthenium nanoparticles by a spontaneous process. The platinum-coated ruthenium nanoparticles are useful as carbon monoxide-tolerant anode electrocatalysts in fuel cells. Also see: Brankovic, S. R., et al, “Pt Submonolayers On Ru Nanoparticles—A Novel Low Pt Loading, High CO Tolerance Fuel Cell Electrocatalyst”, Electrochem. Solid State Lett., 4, p. A217 (2001); and Brankovic, S. R., et al, “Spontaneous Deposition Of Pt On The Ru(0001) Surface”, J. Electroanal. Chem., 503: 99 (2001), which also disclose platinum monolayers on ruthenium nanoparticles.
A method for depositing an atomic monolayer of platinum on palladium nanoparticles was recently reported. See J. Zhang, et al., “Platinum Monolayer Electrocatalysts For O2 Reduction: Pt Monolayer On Pd(111) And On Carbon-Supported Pd Nanoparticles”, J. Phys. Chem. B., 108: 10955 (2004). The method disclosed in Zhang et al. involves first, the electrodeposition of an atomic monolayer of an underpotentially deposited metal such as copper onto palladium nanoparticles. The electrodeposition is followed by contact with a platinum salt solution to initiate a spontaneous redox displacement of the copper atomic monolayer by a platinum monolayer.
The platinum-coated palladium nanoparticles were reported by Zhang et al. to significantly reduce platinum loadings in fuel cell electrocatalysts. In addition, the platinum-coated palladium nanoparticles were reported by Zhang et al. to possess significantly higher catalytic activity for the reduction of oxygen than the corresponding platinum nanoparticles. The higher catalytic activity for the platinum-coated palladium nanoparticles is presumed to be a result of a synergistic effect of palladium on the platinum layer.
It is evident that the platinum-coated palladium nanoparticles hold great promise as a major advance for fuel cell electrocatalysts. In fact, palladium and palladium-alloy particles coated with numerous other metals besides platinum, including the main group and transition metals, are also expected to advance, inter alia, catalysts, electrocatalysts, and other materials.
The method for depositing platinum onto palladium nanoparticles described by Zhang et al. contains several practical limitations. For example, the method of Zhang et al. requires contact of the palladium nanoparticles with an electrode in order to electrodeposit copper onto the palladium nanoparticles. In addition, the method of Zhang et al. generates copper waste during platinum displacement of copper.
None of the art described above discloses a method for depositing a layer of a metal onto a palladium or palladium-alloy particle using a method that is convenient and practical, e.g., that does not require electrodeposition and does not generate waste. Nor does any of the art discussed above disclose a convenient and practical method for the deposition of an atomically thin layer of any of a large variety of metals onto palladium or palladium-alloy particles.
Thus, a convenient and practical method for depositing a layer, particularly an atomically thin layer, of any of a large variety of metals onto palladium and palladium-alloy particles is needed. The present invention relates to such methods.